This invention relates to optical clean-up filters which are optical wavelength filters producing a desired spectral response. To achieve such desired spectral response, the invention uses a plurality of tapered fiber filters in series matching predetermined spectral properties. The invention also includes a method for concatenating the tapered fiber filters to achieve the desired spectral response, after decomposing the latter into individual sine waves.
Tapered optical fiber filters are well known in the art. They are made by tapering a single-mode optical fiber in such a way as to produce an interference between cladding modes, thereby creating a transmission which is wavelength dependent.
One such tapered fiber filter is described in Canadian Patent No. 1,284,282 issued May 21, 1991. It provides a passband filter comprising a plurality of successive biconical tapered portions on a single-mode fiber, such tapered portions having different profiles to produce the desired filtering characteristic.
Also, U.S. Pat. No. 4,946,250 of Aug. 7, 1990 by Gonthier et al., discloses a passband/stopband filter which is formed of two biconical tapers each having a given profile and being separated from each other by a small distance. This enables transmission of one signal of predetermined wavelength while stopping a second signal of a different wavelength.
Moreover, in applicant""s Canadian patent application No. 2,258,140 filed Jan. 6, 1999, there is disclosed a method of making wavelength filters with a sinusoidal response or modulated sine response having any desired filtering amplitude and period of oscillation. The optical fiber filter produced thereby has two coupling regions at the extremities of an elongated central beating zone.
However, the above references do not disclose how to analyze a spectral response and extract the basic sine waves therefrom and then to produce a plurality of filters in the form of suitable fiber tapers and assemble them in line to achieve the desired response in the resulting clean-up filter.
It is an object of the present invention to produce clean-up filters, i.e. filters that are aimed at correcting the wavelength response of optical systems by decomposing the desired response into a plurality of individual responses and then by producing tapered fiber filters specifically designed to fit the individual responses so as to achieve a total response closely matching the desired total response.
Other objects and advantages will be apparent from the following description of the invention.
Optical systems often require a specific predetermined spectral response in order to achieve a desired function or operation. Optical filters are used to help achieve a desired response, but often such filters have predetermined characteristics and are not capable of producing a more complex spectral response when this is desired. The present invention provides such clean-up filters which are capable of producing any desired or predetermined response. To achieve this, the desired filter response is first analyzed with a computer program or algorithm that can automatically or manually simulate independent sine waves into which the desired response can be decomposed and which take the form of the following equation
T=xcex2[1xe2x88x92xcex1 sin2(xcexxe2x88x92xcex0)xcfx80/xcex9]
where:
T is the optical transmission of the filter,
xcex1 is the amplitude of the filter,
xcex2 is the maximum transmission,
xcex is the wavelength,
xcex0 is the reference wavelength or center wavelength of the filter, and
xcex9 is the wavelength period.
The computer program also calculates the product of the function:
F=T1xc3x97T2 . . . xc3x97TN
where:
F is the resulting filter function of the concatenation of the tapers that have the independent transmissions T1 to TN.
Such numerical formula simulates the concatenation of a plurality of tapered individual fiber filters required to achieve the total response F.
The model pre-supposes that the cladding modes between each taper are suppressed, which can be physically achieved in several known ways, for example, by leaving enough fiber length with the protective jacket on between consecutive tapers, by bending the fiber, or by making tapers that are single-mode, and so on. The parameters of the simulation are the parameters or each sine function, namely xcex11, xcex21, xcex1, xcex91, . . . xcex1N, xcex2N, xcexN, xcex9N. These parameters may be adjusted manually or with the aid of a computer program to simulate a response with the smallest deviation from the desired response. The mathematical method used may be based on a minimization of the square of the difference between the model and the desired filter response, but other algorithms may be used or developed.
After thus determining the parameters of the individual tapers, one can realize each individual filter component in practice. The number and type of tapers needed will vary with the desired shape of the total response. For this reason, one must be able to control the parameters mentioned above during the taper fabrication process, in order to achieve the desired total response. When tapering a single-mode fiber, the parameters may be controlled by producing a specific taper slope which itself will be controlled by the size of the heat source used to heat the fiber and by the pulling speed used to produce the desired taper. Using a small flame will cause an abrupt slope to be formed, which will usually result in the coupling of more than two modes, creating a modulated sine response, such as shown in Canadian Patent No. 1,284,282. Such modulated sine response is problematic in the model because it involves the control of additional parameters, such as the amplitude of each mode and the respective phases of the modes.
To avoid this problem, one may produce tapers or filters having a sinusoidal response with only two modes and wherein the amplitude period and phase are suitably controlled by providing two coupling regions at the extremities of an elongated tapered zone. Such filters and the method of their production are disclosed in applicant""s Canadian Patent Application No. 2,258,140 filed Jan. 6, 1999 which is incorporated hereinto by reference. With such taper profiles, one can achieve almost any sine response.
However, when the amplitude of the sine function is less than 50%, a simpler profile can be used, namely a profile such as disclosed in Canadian patent No. 1,284,282, but with a longer taper produced with a wider brush of the flame. The wavelength is then controlled by the length of the taper, i.e. the number of oscillations in the elongation. Because the undesired three or higher order modes are caused by a taper slope that is too steep, one can reduce this effect by reducing the slope. Thus, one can obtain different responses by changing the flame brush width from 0 to a few mm. As one makes tapers with larger and larger brush widths, the modulation amplitude xcex1 will decrease. The appropriate brush width used to obtain a given spectral amplitude xcex1 can thus be determined by successive trials. The two other parameters of the sine response, i.e. the period xcex9 and the peak wavelength xcex, are obtained by controlling the elongation of the taper. During elongation, oscillations in the optical transmission are observed; they correspond to the increase of beat lengths between the LP01 and LP02 modes. As explained in Canadian Patent No. 1,284,282, the number of beats is inversely proportional to the wavelength period; thus, as the taper is elongated, the period decreases. One can thus create periods from 400 nm to less than 1 nm. During fabrication, after the amplitude is set by the proper flame brush width, the elongation process is stopped when the predetermined period and wavelength properties are achieved.
Once a taper is fabricated by either method described above and the desired shape is realized, the taper is bonded to a substrate and suitably packaged for protection, for example in a steel tube. Each taper can be made individually and spliced with other such tapers or components, or they can be made in succession on the same single-mode fiber.
In summary, the present invention provides for an optical clean-up filter with a desired spectral response, which comprises a plurality of tapered fiber filters concatenated in-line on a single-mode optical fiber, said tapered fiber filters having specific wavelength response designs which closely match corresponding specific simulated responses resulting from a decomposition, by means of a computer program or algorithm, of the desired spectral response into individual specific simulated responses, whereby the in-line concatenation of said tapered fiber filters with responses of various specific designs produces the desired specific response in the clean-up filter.
In addition, the present invention provides for a method of manufacturing an optical clean-up filter with a desired spectral response, which comprises:
(a) decomposing the desired spectral response into individual simulated responses using a suitable computer program or algorithm;
(b) manufacturing tapered fiber filters with parameters that closely match the individual simulated responses; and
(c) concatenating the tapered fiber filters on a single-mode fiber to produce the optical clean-up filter with a total response that closely matches the desired spectral response.